When my most recent gar was about 5-6" he reflex attacked a flake by accident but spat it out just as quick. Never heard of a gar going for flakes deliberately.
And if your fish are true gar, the fins will heal in no time if they are healthy to start with. Amazing regeneration.
My gars spend most of their time at the top of the tank, when i put dried shrimp in i also put in flakes for my smaller fish. The shrimp floats as do some of the flakes, therefor they eat a bit of both. They are not going crazy on the flakes but they do munch a few, obviously more of the shrimp tho!
One of my Florida gar (@18") and one of my shortnose gar (@16") both go for flakes on the surface. They seem to think it's freeze dried krill, but soon tire of not getting much flakes with each bite.